As the demand for portability of more and more advanced electronic devices and applications increases, there is a growing need for higher energy and power density energy storage devices. Rechargeable batteries based on Li-ion technology have been very successful at meeting this demand, particularly by penetrating high-end consumer electronic markets to replace lower energy density Ni—Cd and Ni-MH rechargeable batteries. Currently, worldwide annual production of Li-ion rechargeable batteries exceeds 2 billion cells. Lithium ion batteries, in general, are made using a transition metal oxide positive cathode material and a carbon based negative anode material with a micro-porous polyolefin separator between the electrodes. The majority of Li-ion cells (more than 99%) produced today are small size and low capacity cylindrical and prismatic cells (less than 2.5 Ah) though Li-ion batteries are also an attractive technology for emerging larger size, high capacity and high power rechargeable battery applications within the transportation, telecommunication and military markets.
While Li-ion cells offer the greatest rechargeable energy density on the market, they are also very sensitive to the voltage range within which they are cycled, which often limits the applications in which they can be used. In particular, Li-ion cells that are charged beyond a critical upper voltage limit can suffer degraded cycle life performance due to lithium plating at the anode and increases in impedance of the cathode. In the worst overcharge situations, shorts can form in the cell or the cell may suffer from thermal runaway leading to catastrophic failure, venting and explosions. Manufacturers are able to minimize the safety risk of small cells by incorporating expensive external and internal protection devices such as an electronic protection circuit, a disconnect disc, and a polymeric positive-temperature-coefficient resistor (PTC). Unfortunately, for larger cells with greater stored energy, and systems that require high currents, the same types of safety devices generally cannot provide sufficient system wide protection and scaling them up is often prohibitively expensive.
One of the best mechanisms for improving the safety of large cells is the use of a reversible voltage-activated bypass mechanism. Such a mechanism prevents the cell from being charged above (or below) a specific voltage by bypassing the excess charging (or discharging) current around the cell electrodes through a secondary low resistance circuit. Thus the cell electrodes can be prevented from charging (or discharging) outside of the voltage range within which they remain stable enough to reversibly cycle well and are not susceptible to excess heat generation, thermal runaway or catastrophic explosions. Because this mechanism is specifically triggered by the cell voltage and is reversible, it addresses many of the most difficult safety issues of a Li-ion cell by directly preventing cell overcharge and allowing for easier cell balancing in multi-cell packs. For current commercial systems an electronic circuit is used to prevent cell overcharge and to control cell balancing in packs. However, these devices are expensive and are not sufficient to guarantee cell safety and life. Providing the same protection internal to the individual cells is highly desirable. One approach that has had some success is the use of an electrolyte additive compound referred to as a redox shuttle. A redox shuttle acts as an electron shuttle between the anode and cathode of the cell at a specific onset voltage determined by the oxidation voltage of the additive. A redox shuttle provides a voltage activated short within the cell. A number of compounds have been proposed as redox shuttles, though their long-term stability and capability of handling high current densities is often limited.
As disclosed in U.S. Pat. No. 6,228,516, another concept for a reversible internal cell bypass is to use a self-switching voltage activated conductive material to create a bypass circuit. In one embodiment of this concept it was proposed that a self-switching material such as a voltage activated conductive polymer (VACP) be used to directly connect the anode and cathode electrodes. A VACP is a polymer material that can reversibly switch from an insulating state to an electrically conductive state upon oxidation and/or reduction. When the self-switching VACP based material becomes conductive above a certain cell voltage, an electrically conductive path is created between the anode and cathode and the cell is effectively shorted, preventing further increases in the cell voltage. The mechanism can also work for over-discharged cells. When the cell voltage falls back within the normal operation range the voltage activated conductive polymer again becomes insulating, and the cell operates in a normal fashion.
A version of this concept was recently demonstrated and the results reported in Electrochemical and Solid State Letters, 2004, 7(2), A23-26. A self-switching bypass structure for a Li-ion cell was made by coating a voltage activated conductive polymer (VACP), poly(3-butylthiophene) (P3BT), onto a conventional micro-porous polyethylene separator. By their method, the VACP is dissolved in a solvent such as chloroform to form a low viscosity solution. The solution is coated on both sides of a commercial PE or PP micro-porous separator (Celgard 2500). The solution flows into and through the preexisting pores of the polyolefin separator. When the chloroform evaporates it leaves behind a film of VACP on the surface of the separator and a network of solid VACP that has penetrated the existing pores of the separator to connect the two, coated faces of the separator. The use and effect of the VACP coated separator is similar to a standard external electronic bypass circuit though potentially less expensive and more responsive to overcharge conditions. A Li-ion cell was made using a standard LiMn2O4 cathode and Carbon anode laminates with the VACP coated separator sandwiched in between. The VACP coated separator became electrically conductive to generate a short between the anode and cathode electrodes when the cell voltage exceeded the conductive onset voltage of the VACP material, in this case ˜3.4 V. Thus the cell could not be charged beyond this point, preventing cell overcharge or potentially allowing for cell balancing in strings of cells. In this concept demonstration, the maximum bypass current achieved was ˜0.2 mA/cm2, above which the cell voltage would continue to rise.
Prior to coating with the VACP conductive polymer to make a bypass capable separator, the conventional polyethylene (PE) and polypropylene (PP) separators are typically manufactured using a two-step process. The first step is to form a polymer film from the polyolefin material, and the second is to generate pores in the polymer film. The initial polymer film is produced for example by one of two processes: 1) Melt-extrusion through a die, such as T-die, or 2) Blown-film melt-extrusion through a die with an annular shape. The generation of micro-scale pores in these polymer films is mainly done using one of the following three processes: 1) dry-stretch process, 2) wet-extraction process or 3) particle stretch process. To produce the popular tri-layer polypropylene/polyethylene/polypropylene (PP/PE/PP) separator, three common processes are currently used: 1) Producing three individual porous films such as PP, PE and PP followed by lamination, 2) Producing three individual non-porous films followed by lamination and then generating micro-scale pores using one of the pore generating processes listed above, or 3) Co-extruding the three films together and again generating micro-scale pores using one of the pore generating processes listed above. Although all of these processes are somewhat different from each other, each of them is used to produce separators that are widely used in commercial Li-ion battery products and are suitable for making a bypass capable separator by utilizing a subsequent coating method to apply a VACP layer onto the separator.
The current state of the art bypass separators, wherein a conventional separator is coated with a solution of VACP, suffers from a number of problems that are detrimental to the utility of the bypass separator in an electrochemical cell. For example, the coating process necessarily clogs the pores of the separator with the VACP phase to provide an electrical current path from one face of the separator to the other. The presence of the VACP material in the separator pores leads to a higher cell impedance and lower power density of the cell. Reducing the amount of VACP present in the pores to minimize this effect in turn reduces the maximum bypass current density that the bypass separator can handle. In other words, the cross-sectional area of the conductive phase path from one face of the separator to the other face of the separator is generally quite low and the resulting impedance of the conductive path is quite high. Another issue is that the coating process results in the bulk of the VACP being present on the surface of the two faces of the separator film where it contributes very little to the current bypass capability of the separator. Because VACP materials are typically more expensive than the materials used to make the separator, it is preferred that the amount of the VACP material be minimized.
The coating process used to make the current state of the art bypass separators itself has many limitations. A major limitation with the coating process is that the VACP phase must be soluble in a solvent that can be used to coat the separator. Unfortunately, the low molecular weight VACP materials that are soluble and can be coated are often soluble or semi-soluble in the electrochemical cell electrolyte, greatly limiting their stability and long term life in a real cell. Furthermore, the additional coating process may add significant cost to the separator due to the equipment and time required to coat the separator and due to the use of large amounts of solvents needed.
While the current approach of utilizing a conventional Li-ion separator coated with a layer of VACP as an internal electrochemical cell, voltage-activated current bypass device is promising, there are still numerous performance, cost, engineering, stability and processing problems with such a bypass separator and with the current method and materials for making such a separator. Accordingly there exists a need for an improved bypass separator and the methods for its manufacture.